To prevent malfunction and ensure the stability of electronic control, some devices for controlling driving of a load, driven by a large current, by a control integrated circuit (IC) are provided with low voltage protection function. This function is such that when it is detected that a power supply voltage has dropped to some degree, drive control by the control IC is stopped.
U.S. Pat. No. 6,912,141 (JP 2003-79146A) discloses an example of such a device having the low voltage protection function. As shown in FIG. 10, the device for driving of a load mounted in, for example, a vehicle as an example of a control device is provided with the above protection function.
A control unit 1 is so constructed that its power supply terminal (+B) and ground terminal (GND) are connected with a battery 2 of a vehicle. The power supply voltage is, for example, 12V or so. In the control unit 1, a control IC 4 is connected between power wires 3p, 3n and further a series circuit of a p-channel MOSFET 5 and a load 6 is connected. A flywheel diode 7 is connected in parallel with the load 6. A smoothing inductor 8 is placed in the power wire 3p. 
Upon receiving a control signal supplied from an external electronic control unit (ECU) 9, the control IC 4 outputs a gate signal to the FET 5 to control driving of the load 6. However, the ECU 9 exists outside the control unit 1. The control IC 4 monitors the voltage level of power internally supplied to itself, and is so constructed that when that voltage drops to 6V or so, for example, it stops driving of the load 6.
However, in this device, the power wire 3p constructed of a bus bar and the wiring pattern, thick film conductor wiring, and the inductor 8 on a printed board, and the like have a resistive component R. When a load current may continue to be excessively increased, for example, a voltage drop occurs in correspondence with the current. The power supply voltage to the load 6 drops with increase in the load current. When the low voltage protection is triggered, driving of the load 6 is stopped and the load current does not flow any more.
Thus, voltage drop by the resistive component R in the power supply path is eliminated and the power supply voltage is increased again. As a result, drive control is resumed by the control IC 4. When this control is resumed, low voltage protection is triggered by voltage drop again. Therefore, hunting occurs and control is repeatedly and alternately stopped and resumed at short time intervals as shown in FIG. 11. When the low voltage protection is carried out, generally, hysteresis characteristics are provided in voltage detection for preventing erroneous detection. However, the hysteresis width is set to a value equal to or lower than the voltage range specification of a product. No consideration is given to the amount of voltage drop caused by wiring resistance.
When such hunting occurs, some kinds of loads to be driven may produce abnormal noise, and this brings discomfort to the user.